This application proposes a line of investigation into the in vivo mechanisms that control somatostatin (SRIF-14) biosynthesis in the rat hypothalamus. SRIF-14 is believed to be a hypothalamic hormone that influences the secretion of pituitary growth hormone (GH); it may also function as a neurotransmitter elsewhere in brain. While some information is available concerning possible precursor peptides from which SRIF-14 may be derived, very little is known about the dynamics of SRIF-14 synthesis in vivo. Recently, we have developed a series of high-performance liquid chromatographic (HPLC) separations that allow us to isolate SRIF-14 and its presumed immediate presursor, somatostatin-28 (SRIF-28), and techniques for obtaining incorporation of 35S-cysteine in vivo into these peptides following amino acid injection into the their ventricle. Coupled with a radioimmunoassay that allows measurement of endogenous levels of both SRIF-14 and SRIF-28 following HPLC separation, these methods will allow us to obtain estimates for the first time of SRIF-14 and SRIF-28 biosynthesis in vivo in brain. We propose in this application to study several issues relating to SRIF biosynthesis using thes newly-developed techniques. Specifically, we hope to determine [1] if 35S-cysteine-SRIF-14 (and -28) is synthesized via the neuron's protein sythetic machinery, if the peptide is derived from larger molecular weight precursors (like SRIF-28, and a proposed 12,000 mw precursor), and if conversion from precursors to SRIF-14 occurs during transit from cell body to nerve terminal; [2] if SRIF-14 synthesis is subject to control by circulating (or released) GH, or SRIF-14 or SRIF-28; [3] if SRIF biosynthesis changes under physiologic and pathophysiologic conditions shown previously to alter hypothalamic SRIF content and blood GH levels; and [4] if the administration of selected neuropharmacologic agents thought to influence GH secretion via effects on hypothalamic SRIF release modify hypothalamic SRIF-14 and SRIF-28 synthesis (and presumably release) in the manner predicted by these previous findings. If successful, these studies should provide important information about the in vivo regulation of the synthesis of a brain neuropeptide thought to function both as a hormone and possible also as a neurotransmitter.